In this plasmid, another DNA sequence produce the GPF (green fluorescence protien) that results in the bacteria fluorescing under UN light. The there gene araC regulates the expression of GFP.
Look up lac and trp operons. they are both fine examples of negative and positive controls, respectively.
However, to answer your question in short… do you think it would be prudent for a bacterium to produce an enzyme for a sugar that it isn’t ingesting?
